|Publication number||US7800956 B2|
|Application number||US 12/163,073|
|Publication date||Sep 21, 2010|
|Filing date||Jun 27, 2008|
|Priority date||Jun 27, 2008|
|Also published as||CN102138181A, CN102138181B, EP2301033A1, EP2301033B1, US20090323429, WO2009158350A1|
|Publication number||12163073, 163073, US 7800956 B2, US 7800956B2, US-B2-7800956, US7800956 B2, US7800956B2|
|Inventors||Dana Lee, Deepanshu Dutta, Yingda Dong|
|Original Assignee||Sandisk Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (36), Non-Patent Citations (1), Referenced by (21), Classifications (9), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
The present invention relates to non-volatile memory.
2. Description of the Related Art
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrically Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories. With flash memory, also a type of EEPROM, the contents of the whole memory array, or of a portion of the memory, can be erased in one step, in contrast to the traditional, full-featured EEPROM.
Both the traditional EEPROM and the flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage (VTH) of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory element can be programmed/erased between two states, e.g., an erased state and a programmed state. Such a flash memory device is sometimes referred to as a binary flash memory device because each memory element can store one bit of data.
A multi-state (also called multi-level) flash memory device is implemented by identifying multiple distinct allowed/valid programmed threshold voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits encoded in the memory device. For example, each memory element can store two bits of data when the element can be placed in one of four discrete charge bands corresponding to four distinct threshold voltage ranges.
Typically, a program voltage VPGM applied to the control gate during a program operation is applied as a series of pulses that increase in magnitude over time. In one possible approach, the magnitude of the pulses is increased with each successive pulse by a predetermined step size, e.g., 0.2-0.4 V. VPGM can be applied to the control gates of flash memory elements. In the periods between the program pulses, verify operations are carried out. That is, the programming level of each element of a group of elements being programmed in parallel is read between successive programming pulses to determine whether it is equal to or greater than a verify level to which the element is being programmed. For arrays of multi-state flash memory elements, a verification step may be performed for each state of an element to determine whether the element has reached its data-associated verify level. For example, a multi-state memory element capable of storing data in four states may need to perform verify operations for three compare points.
Moreover, when programming an EEPROM or flash memory device, such as a NAND flash memory device in a NAND string, typically VPGM is applied to the control gate and the bit line is grounded, causing electrons from the channel of a cell or memory element, e.g., storage element, to be injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory element is raised so that the memory element is considered to be in a programmed state. More information about such programming can be found in U.S. Pat. Nos. 6,859,397 and 6,917,542, both of which are incorporated herein by reference in their entirety.
However, one issue which continues to be problematic is program disturb. Program disturb can occur at inhibited NAND strings during programming of other NAND strings, and sometimes at the programmed NAND string itself. Program disturb occurs when the threshold voltage of an unselected non-volatile storage element is shifted due to programming of other non-volatile storage elements. Program disturb can occur on previously programmed storage elements as well as erased storage elements that have not yet been programmed. Multi-pass programming techniques can reduce program disturb by programming storage elements more gradually. However, programming time is increased.
The present invention addresses the above and other issues by providing a programming technique which reduces program disturb in a non-volatile storage system while also reducing programming time.
In one embodiment, a method for operating non-volatile storage includes performing programming operations on a first set of storage elements using a first verify level less an offset to distinguish slower and faster programming storage elements, while locking out at least a second set of storage elements from being programmed. The method further includes locking out the faster programming storage elements while continuing programming of the slower programming storage elements using the first verify level and while continuing to lock out the at least a second set of storage elements. The method further includes locking out the slower programming storage elements while resuming programming of the faster programming storage elements using the first verify level and while programming the at least a second set of storage elements using at least a second verify level which is less than the first verify level less the offset.
In another embodiment, a method for operating non-volatile storage includes programming storage elements which are intended to be programmed to a first data state associated with a first verify level, while locking out from programming other storage elements which are intended to be programmed to at least second and third data states associated with second and third verify levels, respectively, which are below the first verify level. The method further includes, during the programming, distinguishing slower and faster programming storage elements among the storage elements which are intended to be programmed to the first data state, and locking out the faster programming storage elements while continuing programming of the slower programming storage elements. The method further includes subsequently programming the at least a second set of storage elements to the at least second and third data states, while resuming programming of the faster programming storage elements to the first data state, and applying a programming condition for the continuing programming of the slower programming storage elements which differs from a programming condition for the resuming programming of the faster programming storage elements.
In another embodiment, a method for operating non-volatile storage includes: a) programming storage elements which are intended to be programmed to a first data state associated with a first verify level, while locking out from programming other storage elements which are intended to be programmed to at least second and third data states associated with second and third verify levels, respectively, which are below the first verify level. The method further includes: b) subsequently programming the at least a second set of storage elements, where step a) uses a programming condition which differs from a programming condition of step b).
In another embodiment, a non-volatile storage system includes a first set of storage elements, and at least one control circuit in communication with the first set of storage elements. The at least one control circuit: (a) performs programming operations on the first set of storage elements using a first verify level less an offset to distinguish slower and faster programming storage elements, while locking out at least a second set of storage elements from being programmed, (b) locks out the faster programming storage elements while continuing programming of the slower programming storage elements using the first verify level and while continuing to lock out the at least a second set of storage elements, and (c) locks out the slower programming storage elements while resuming programming of the faster programming storage elements using the first verify level and while programming the at least a second set of storage elements using at least a second verify level which is less than the first verify level less the offset.
Corresponding methods, systems and computer- or processor-readable storage devices for performing the methods provided herein may be provided.
The present invention provides a programming technique which reduces program disturb in a non-volatile storage system while also reducing programming time.
One example of a memory system suitable for implementing the present invention uses the NAND flash memory structure, which includes arranging multiple transistors in series between two select gates. The transistors in series and the select gates are referred to as a NAND string.
For example, NAND string 320 includes select gates 322 and 327, and storage elements 323-326, NAND string 340 includes select gates 342 and 347, and storage elements 343-346, NAND string 360 includes select gates 362 and 367, and storage elements 363-366. Each NAND string is connected to the source line by its select gates (e.g., select gates 327, 347 or 367). A selection line SGS is used to control the source side select gates. The various NAND strings 320, 340 and 360 are connected to respective bit lines 321, 341 and 361, by select transistors in the select gates 322, 342, 362, etc. These select transistors are controlled by a drain select line SGD. In other embodiments, the select lines do not necessarily need to be in common among the NAND strings; that is, different select lines can be provided for different NAND strings. Word line WL3 is connected to the control gates for storage elements 323, 343 and 363. Word line WL2 is connected to the control gates for storage elements 324, 344 and 364. Word line WL1 is connected to the control gates for storage elements 325, 345 and 365. Word line WL0 is connected to the control gates for storage elements 326, 346 and 366. As can be seen, each bit line and the respective NAND string comprise the columns of the array or set of storage elements. The word lines (WL3, WL2, WL1 and WL0) comprise the rows of the array or set. Each word line connects the control gates of each storage element in the row. Or, the control gates may be provided by the word lines themselves. For example, word line WL2 provides the control gates for storage elements 324, 344 and 364. In practice, there can be thousands of storage elements on a word line.
Each storage element can store data. For example, when storing one bit of digital data, the range of possible threshold voltages (VTH) of the storage element is divided into two ranges which are assigned logical data “1” and “0.” In one example of a NAND type flash memory, the VTH is negative after the storage element is erased, and defined as logic “1.” The VTH after a program operation is positive and defined as logic “0.” When the VTH is negative and a read is attempted, the storage element will turn on to indicate logic “1” is being stored. When the VTH is positive and a read operation is attempted, the storage element will not turn on, which indicates that logic “0” is stored. A storage element can also store multiple levels of information, for example, multiple bits of digital data. In this case, the range of VTH value is divided into the number of levels of data. For example, if four levels of information are stored, there will be four VTH ranges assigned to the data values “11”, “10”, “01”, and “00.” In one example of a NAND type memory, the VTH after an erase operation is negative and defined as “11”. Positive VTH values are used for the states of “10”, “01”, and “00.” The specific relationship between the data programmed into the storage element and the threshold voltage ranges of the element depends upon the data encoding scheme adopted for the storage elements. For example, U.S. Pat. Nos. 6,222,762 and 7,237,074, both of which are incorporated herein by reference in their entirety, describe various data encoding schemes for multi-state flash storage elements.
Relevant examples of NAND type flash memories and their operation are provided in U.S. Pat. Nos. 5,386,422, 5,570,315, 5,774,397, 6,046,935, 6,456,528 and 6,522,580, each of which is incorporated herein by reference.
When programming a flash storage element, a program voltage is applied to the control gate of the storage element, and the bit line associated with the storage element is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the VTH of the storage element is raised. To apply the program voltage to the control gate of the storage element being programmed, that program voltage is applied on the appropriate word line. As discussed above, one storage element in each of the NAND strings share the same word line. For example, when programming storage element 324 of
However, program disturb can occur at inhibited NAND strings during programming of other NAND strings, and sometimes at the programmed NAND string itself. Program disturb occurs when the threshold voltage of an unselected non-volatile storage element is shifted due to programming of other non-volatile storage elements. Program disturb can occur on previously programmed storage elements as well as erased storage elements that have not yet been programmed. Various program disturb mechanisms can limit the available operating window for non-volatile storage devices such as NAND flash memory.
For example, if NAND string 320 is inhibited (e.g., it is an unselected NAND string which does not contain a storage element which is currently being programmed) and NAND string 340 is being programmed (e.g., it is a selected NAND string which contains a storage element which is currently being programmed), program disturb can occur at NAND string 320. For example, if a pass voltage, VPASS, is low, the channel of the inhibited NAND string is not well boosted, and a selected word line of the unselected NAND string can be unintentionally programmed. In another possible scenario, the boosted voltage can be lowered by Gate Induced Drain Leakage (GIDL) or other leakage mechanisms, resulting in the same problem. Other effects, such as shifts in the VTH of a charge storage element due to capacitive coupling with other neighboring storage elements that are programmed later, can also contribute to program disturb.
During a program operation, a control gate voltage VPGM is provided on a selected word line, in this example, WL3, which is associated with storage element 414. Further, recall that the control gate of a storage element may be provided as a portion of the word line. For example, WL0, WL1, WL2, WL3, WL4, WL5, WL6 and WL7 can extend via the control gates of storage elements 408, 410, 412, 414, 416, 418, 420 and 422, respectively. A pass voltage, VPASS is applied to the remaining word lines associated with NAND string 400, in one possible boosting scheme. Some boosting schemes apply different pass voltages to different word lines. VSGS and VSGD are applied to the select gates 406 and 424, respectively.
A first part of a first programming pass is depicted in
At this time, a second part of the first programming pass begins. As depicted in
In a second programming pass, the series of program pulses are again applied to the cells to be programmed, e.g., via a selected word line, as depicted in
The above approach involves repeating the entire sequence of program pulses in separate passes, depicted by
One approach involves skipping many of the repeated program pulses and thus reducing programming time while preserving the major benefits of a multi-pass programming technique, as explained now in further detail.
In a second phase of the first programming pass, a threshold voltage detection is performed to separate fast C cells from slow C cells. Thus, the fast and slow memory cells are identified or grouped. The C cells are programmed from the LM state to a range of threshold voltages between values VC1 and VC2, as indicated in
Alternatively, the decision to conclude phase 3 can be determined adaptively on a “per-case” basis as a function of the number of C cells, or the portion of C cells in a set of C cells such as in a word line or block, for which VTH>VV-CLL. For example, phase 2 can be completed once the first C cell reaches VTH>VV-CLL. It is also possible to complete phase 2 after some fixed number of additional program pulses after a pre-determined number or portion of cells reach VTH>VV-CLL.
After phase 2 is completed, the threshold voltage of the slow C cells will be in a range 610 between VC1 and VV-CLL, and the threshold voltage of the fast C cells will be in a range 620 between VV-CLL and VC2. Note that this example indicates that the distribution of slow and fast C cells is about 50% and 50%, respectively. However, it is not necessary for the number of fast and slow C cells to be equal. Further, regarding the range of the threshold voltage distribution, the example indicates that the low end of the distribution at VC1 falls within the LM distribution. However, this is not necessary as the low end of the distribution at VC1 may be above the LM distribution. Further, the high end of the C cell distribution at VC2 is indicated as being at the high end of the final C state distribution, but it may alternatively be lower.
After phase 2, a third phase of the first programming pass occurs in which the fast C cells which were identified in phase 2 are temporarily locked out from programming, and programming of the slow C cells continues, as indicated in
Moreover, the first program pulse of phase 3 may exceed the last program pulse of phase 2 by ΔV4, while a step size of ΔV5 is used for subsequent pulses in phase 3. In one approach, ΔV4 is greater than ΔV2 and ΔV1. ΔV5 may be greater than ΔV1 and comparable to ΔV3. In one approach, ΔV4 is equal to or approximately equal to VV-C−VV-CLL, that is, the difference between the C state verify level and the low C state verify level. Generally, the program pulse level can be sharply stepped up in phase 3 compare to prior pulses because it is known that slow cells are being programmed. Such cells are relatively stubborn and therefore require a higher and/or longer duration of program pulse to be applied to their control gates via a selected word line in order to continue to elevate their threshold voltage toward the final intended state. Thus, another option includes extending the duration of the program pulses in addition to, or in lieu of, raising the program pulse level, in phase 3.
After phase 3, the first programming pass is complete, and a second program pass is subsequently performed. Thus, the latter portion of the first programming pass involves the application of a number of program pulses during which only cells targeted for the last, highest state are selected. The starting and final VPGM values, along with the increments and step sizes, are parameters to be optimized during characterization.
The second program pass is explained in conjunction with
The previously locked out fast C cells (represented by VTH distribution 630) are restored so that they are programmed together with the A and B cells, as depicted in
Since there are no longer any slow-C cells, and only fast C cells remain to be programmed, the second programming pass should finish sooner than in the approach of
Optionally, the techniques mentioned herein can be modified to include a coarse/fine programming process, as depicted in
For example, in the first programming pass, in which the slow C cells are programmed to the C state (e.g.,
Further, other modifications are applicable to any of the above-mentioned techniques, including those discussed in connection with
Step 816 includes performing a verify at VV-CLL and temporarily locking out the C cells whose VTH>VV-CLL. These locked out cells are the fast C cells. Decision step 818 determines if some number N2 of program pulses have been applied, or some number or portion N3 of C cells have reached VV-CLL. Another option is to determine whether some number or portion of C cells have reached VV-CLL and some number of additional program pulses have been applied. If neither case is true, a next program pulse is applied at step 820, incrementing the prior pulse by ΔV3. The process then loops back to step 816. If decision step 818 is true, phase 3 of the first programming pass begins at step 822. Here, all cells are temporarily locked out from programming except the slow C cells (e.g., the A, B and fast C cells are locked out). Step 824 includes applying a program pulse which is incremented from the prior pulse by ΔV4.
Step 826 includes performing a verify at VV-C and permanently locking out the slow C cells for which VTH>VV-C. The permanent lock out refers to a lockout which extends through the current programming operation. Decision step 828 determines if all slow C cells have reached state C or whether some number N4 of program pulses have been applied. Note that the number of program pulses referred to herein (e.g., N1-N8) can be expressed in terms of program pulses in the current phase and/or total number of program pulses in the current programming pass. If neither case is true in decision step 828, a next program pulse is applied at step 830, incrementing the prior pulse by ΔV5. The process then loops back to step 826. If decision step 828 is true, phase 4, and the second programming pass, begins at step 832. Here, the A, B and fast C cells are restored from their temporary lockout so that they can be programmed. The process continues at
Step 862 includes performing verify operations at VV-A, VV-B and VV-C and permanently locking out the A, B and C cells which reach their intended state (e.g., for which VTH>VV-A, VTH>VV-B and VTH>VV-C, respectively). Decision step 864 determines if all A, B and C cells have reached the intended state A, B or C, or whether some number N7 of program pulses have been applied. If neither case is true, a next program pulse is applied at step 866, incrementing the prior pulse by ΔV1. The process then loops back to step 862. If decision step 864 is true, phase 7, the final phase of the second programming pass, begins at step 868, in which case the next program pulse, using an increment of ΔV1, is applied at step 870. Step 872 includes performing verify operations at VV-B and VV-C and permanently locking out the B and C cells which reach their intended state (e.g., for which VTH>VV-B and VTH>VV-C, respectively). Decision step 874 determines if all B and C cells have reached state B and C, respectively, or whether some number N8 of program pulses have been applied. If neither case is true, a next program pulse is applied at step 876, incrementing the prior pulse by ΔV1. The process then loops back to step 872. If decision step 874 is true, the programming is concluded at step 878.
Step 920 includes starting a first programming pass. Step 922 includes programming the cells to an intermediate state (e.g., the LM state). Step 924 includes programming the high state cells and locking out the fast high state cells after they are detected. Step 926 includes programming the slow high state cells. Step 928 includes starting a second programming pass. Step 930 includes programming the remaining cells, including the fast high state cells.
Case #1 involves 13 verify operations at VV-A and/or VV-AL, 13 verify operations at VV-B and/or VV-BL and 11 verify operations at VV-C and/or VV-CL, for a total of 63 verify operations. Additionally, 22 program loops (e.g., program pulses) are applied. A total of 85 operations are thus performed. Case #2 involves 11 operations at VV-C, 13 operations at VV-A and/or VV-AL and 13 verify operations at VV-B and/or VV-BL, for a total of 63 verify operations. Additionally, 30 program pulses (in total, over two programming passes) are applied. A total of 93 operations are thus performed. Case #3 involves 4 verify operations at VV-CLL, 13 verify operations at VV-A and/or VV-AL, 13 verify operations at VV-B and/or VV-BL and 8 verify operations at VV-CL and/or VV-C, for a total of 65 verify operations. Additionally, 24 program pulses (in total, over two programming passes) are applied. A total of 89 operations are thus performed. Thus, case #3 saves the time involved in applying six additional program pulses compared to case #2. This savings is offset slightly by the need for two additional verify operations, but the overall benefit is still significant. The time savings which may be realized may be even greater in other applications.
The array of storage elements is divided into a large number of blocks of storage elements. As is common for flash EEPROM systems, the block is the unit of erase. That is, each block contains the minimum number of storage elements that are erased together. Each block is typically divided into a number of pages. A page is a unit of programming. In one embodiment, the individual pages may be divided into segments and the segments may contain the fewest number of storage elements that are written at one time as a basic programming operation. One or more pages of data are typically stored in one row of storage elements. A page can store one or more sectors. A sector includes user data and overhead data. Overhead data typically includes an Error Correction Code (ECC) that has been calculated from the user data of the sector. A portion of the controller (described below) calculates the ECC when data is being programmed into the array, and also checks it when data is being read from the array. Alternatively, the ECCs and/or other overhead data are stored in different pages, or even different blocks, than the user data to which they pertain.
A sector of user data is typically 512 bytes, corresponding to the size of a sector in magnetic disk drives. Overhead data is typically an additional 16-20 bytes. A large number of pages form a block, anywhere from 8 pages, for example, up to 32, 64, 128 or more pages. In some embodiments, a row of NAND strings comprises a block.
Memory storage elements are erased in one embodiment by raising the p-well to an erase voltage (e.g., 14-22 V) for a sufficient period of time and grounding the word lines of a selected block while the source and bit lines are floating. Due to capacitive coupling, the unselected word lines, bit lines, select lines, and c-source are also raised to a significant fraction of the erase voltage. A strong electric field is thus applied to the tunnel oxide layers of selected storage elements and the data of the selected storage elements are erased as electrons of the floating gates are emitted to the substrate side, typically by Fowler-Nordheim tunneling mechanism. As electrons are transferred from the floating gate to the p-well region, the threshold voltage of a selected storage element is lowered. Erasing can be performed on the entire memory array, separate blocks, or another unit of storage elements.
The control circuitry 1210 cooperates with the read/write circuits 1265 to perform memory operations on the memory array 1100. The control circuitry 1210 includes a state machine 1212, an on-chip address decoder 1214, and a power control module 1216. The state machine 1212 provides chip-level control of memory operations, including controlling pre-charging. The on-chip address decoder 1214 provides an address interface between that used by the host or a memory controller to the hardware address used by the decoders 1230 and 1260. The power control module 1216 controls the power and voltages supplied to the word lines and bit lines during memory operations.
In some implementations, some of the components of
Sense module 1280 comprises sense circuitry 1270 that determines whether a conduction current in a connected bit line is above or below a predetermined threshold level. Sense module 1280 also includes a bit line latch 1282 that is used to set a voltage condition on the connected bit line. For example, a predetermined state latched in bit line latch 1282 will result in the connected bit line being pulled to a state designating program inhibit (e.g., 1.5-3 V).
Common portion 1290 comprises a processor 1292, a set of data latches 1294 and an I/O Interface 1296 coupled between the set of data latches 1294 and data bus 1220. Processor 1292 performs computations. For example, one of its functions is to determine the data stored in the sensed storage element and store the determined data in the set of data latches. The set of data latches 1294 is used to store data bits determined by processor 1292 during a read operation. It is also used to store data bits imported from the data bus 1220 during a program operation. The imported data bits represent write data meant to be programmed into the memory. I/O interface 1296 provides an interface between data latches 1294 and the data bus 1220.
During read or sensing, the operation of the system is under the control of state machine 1212 that controls the supply of different control gate voltages to the addressed storage element. As it steps through the various predefined control gate voltages corresponding to the various memory states supported by the memory, the sense module 1280 may trip at one of these voltages and an output will be provided from sense module 1280 to processor 1292 via bus 1272. At that point, processor 1292 determines the resultant memory state by consideration of the tripping event(s) of the sense module and the information about the applied control gate voltage from the state machine via input lines 1293. It then computes a binary encoding for the memory state and stores the resultant data bits into data latches 1294. In another embodiment of the core portion, bit line latch 1282 serves double duty, both as a latch for latching the output of the sense module 1280 and also as a bit line latch as described above.
It is anticipated that some implementations will include multiple processors 1292. In one embodiment, each processor 1292 will include an output line (not depicted) such that each of the output lines is wired-OR'd together. In some embodiments, the output lines are inverted prior to being connected to the wired-OR line. This configuration enables a quick determination during the program verification process of when the programming process has completed because the state machine receiving the wired-OR can determine when all bits being programmed have reached the desired level. For example, when each bit has reached its desired level, a logic zero for that bit will be sent to the wired-OR line (or a data one is inverted). When all bits output a data 0 (or a data one inverted), then the state machine knows to terminate the programming process. Because each processor communicates with eight sense modules, the state machine needs to read the wired-OR line eight times, or logic is added to processor 1292 to accumulate the results of the associated bit lines such that the state machine need only read the wired-OR line one time. Similarly, by choosing the logic levels correctly, the global state machine can detect when the first bit changes its state and change the algorithms accordingly.
During program or verify, the data to be programmed is stored in the set of data latches 1294 from the data bus 1220. The program operation, under the control of the state machine, comprises a series of programming voltage pulses applied to the control gates of the addressed storage elements. Each programming pulse is followed by a read back (verify) to determine if the storage element has been programmed to the desired memory state. Processor 1292 monitors the read back memory state relative to the desired memory state. When the two are in agreement, the processor 1292 sets the bit line latch 1282 so as to cause the bit line to be pulled to a state designating program inhibit. This inhibits the storage element coupled to the bit line from further programming even if programming pulses appear on its control gate. In other embodiments the processor initially loads the bit line latch 1282 and the sense circuitry sets it to an inhibit value during the verify process.
Data latch stack 1294 contains a stack of data latches corresponding to the sense module. In one embodiment, there are three data latches per sense module 1280. In some implementations (but not required), the data latches are implemented as a shift register so that the parallel data stored therein is converted to serial data for data bus 1220, and vice versa. In the preferred embodiment, all the data latches corresponding to the read/write block of m storage elements can be linked together to form a block shift register so that a block of data can be input or output by serial transfer. In particular, the bank of read/write modules is adapted so that each of its set of data latches will shift data in to or out of the data bus in sequence as if they are part of a shift register for the entire read/write block.
Additional information about the structure and/or operations of various embodiments of non-volatile storage devices can be found in U.S. Pat. Nos. 7,196,931, 7,023,736, 7,046,568, 7,196,928 and 7,327,619. All five of the immediately above-listed patent documents are incorporated herein by reference in their entirety.
In the example provided, four storage elements are connected in series to form a NAND string. Although four storage elements are shown to be included in each NAND string, more or less than four can be used (e.g., 16, 32, 64 or another number). One terminal of the NAND string is connected to a corresponding bit line via a drain select gate (connected to select gate drain lines SGD), and another terminal is connected to c-source via a source select gate (connected to select gate source line SGS).
In another embodiment, referred to as an odd-even architecture (architecture 1500), the bit lines are divided into even bit lines (BLe) and odd bit lines (BLo). In the odd/even bit line architecture, storage elements along a common word line and connected to the odd bit lines are programmed at one time, while storage elements along a common word line and connected to even bit lines are programmed at another time. In each block, in this example, there are 8,512 columns that are divided into even columns and odd columns. In this example, four storage elements are shown connected in series to form a NAND string. Although four storage elements are shown to be included in each NAND string, more or fewer than four storage elements can be used.
During one configuration of read and programming operations, 4,256 storage elements are simultaneously selected. The storage elements selected have the same word line and the same kind of bit line (e.g., even or odd). Therefore, 532 bytes of data, which form a logical page, can be read or programmed simultaneously, and one block of the memory can store at least eight logical pages (four word lines, each with odd and even pages). For multi-state storage elements, when each storage element stores two bits of data, where each of these two bits are stored in a different page, one block stores sixteen logical pages. Other sized blocks and pages can also be used.
For either the ABL or the odd-even architecture, storage elements can be erased by raising the p-well to an erase voltage (e.g., 20 V) and grounding the word lines of a selected block. The source and bit lines are floating. Erasing can be performed on the entire memory array, separate blocks, or another unit of the storage elements which is a portion of the memory device. Electrons are transferred from the floating gates of the storage elements to the p-well region so that the VTH of the storage elements becomes negative.
In the read and verify operations, the select gates (SGD and SGS) are connected to a voltage in a range of 2.5-4.5 V and the unselected word lines (e.g., WL0, WL1 and WL3, when WL2 is the selected word line) are raised to a read pass voltage, VREAD, (typically a voltage in the range of 4.5 to 6 V) to make the transistors operate as pass gates. The selected word line WL2 is connected to a voltage, a level of which is specified for each read and verify operation in order to determine whether a VTH of the concerned storage element is above or below such level. For example, in a read operation for a two-level storage element, the selected word line WL2 may be grounded, so that it is detected whether the VTH is higher than 0 V. In a verify operation for a two level storage element, the selected word line WL2 is connected to 0.8 V, for example, so that it is verified whether or not the VTH has reached at least 0.8 V. The source and p-well are at 0 V. The selected bit lines, assumed to be the even bit lines (BLe), are pre-charged to a level of, for example, 0.7 V. If the VTH is higher than the read or verify level on the word line, the potential level of the bit line (BLe) associated with the storage element of interest maintains the high level because of the non-conductive storage element. On the other hand, if the VTH is lower than the read or verify level, the potential level of the concerned bit line (BLe) decreases to a low level, for example, less than 0.5 V, because the conductive storage element discharges the bit line. The state of the storage element can thereby be detected by a voltage comparator sense amplifier that is connected to the bit line.
The erase, read and verify operations described above are performed according to techniques known in the art. Thus, many of the details explained can be varied by one skilled in the art. Other erase, read and verify techniques known in the art can also be used.
Each distinct threshold voltage range corresponds to predetermined values for the set of data bits. The specific relationship between the data programmed into the storage element and the threshold voltage levels of the storage element depends upon the data encoding scheme adopted for the storage elements. For example, U.S. Pat. Nos. 6,222,762 and 7,237,074, both of which are incorporated herein by reference in their entirety, describe various data encoding schemes for multi-state flash storage elements. In one embodiment, data values are assigned to the threshold voltage ranges using a Gray code assignment so that if the threshold voltage of a floating gate erroneously shifts to its neighboring physical state, only one bit will be affected. One example assigns “11” to threshold voltage range E (state E), “10” to threshold voltage range A (state A), “00” to threshold voltage range B (state B) and “01” to threshold voltage range C (state C). However, in other embodiments, Gray code is not used. Although four states are shown, the present invention can also be used with other multi-state structures including those that include more or less than four states.
Three read reference voltages, Vra, Vrb and Vrc, are also provided for reading data from storage elements. By testing whether the threshold voltage of a given storage element is above or below Vra, Vrb and Vrc, the system can determine the state, e.g., programming condition, the storage element is in.
Further, three verify reference voltages, Vva, Vvb and Vvc, are provided. When programming storage elements to state A, the system will test whether those storage elements have a threshold voltage greater than or equal to Vva. When programming storage elements to state B, the system will test whether the storage elements have threshold voltages greater than or equal to Vvb. When programming storage elements to state C, the system will determine whether storage elements have their threshold voltage greater than or equal to Vvc.
In one embodiment, known as full sequence programming, storage elements can be programmed from the erase state E directly to any of the programmed states A, B or C. For example, a population of storage elements to be programmed may first be erased so that all storage elements in the population are in erased state E. A series of programming pulses such as depicted by the control gate voltage sequence of
In a first programming pass, the storage element's threshold voltage level is set according to the bit to be programmed into the lower logical page. If that bit is a logic “1,” the threshold voltage is not changed since it is in the appropriate state as a result of having been earlier erased. However, if the bit to be programmed is a logic “0,” the threshold level of the storage element is increased to be state A, as shown by arrow 1700. That concludes the first programming pass.
In a second programming pass, the storage element's threshold voltage level is set according to the bit being programmed into the upper logical page. If the upper logical page bit is to store a logic “1,” then no programming occurs since the storage element is in one of the states E or A, depending upon the programming of the lower page bit, both of which carry an upper page bit of “1.” If the upper page bit is to be a logic “0,” then the threshold voltage is shifted. If the first pass resulted in the storage element remaining in the erased state E, then in the second phase the storage element is programmed so that the threshold voltage is increased to be within state C, as depicted by arrow 1720. If the storage element had been programmed into state A as a result of the first programming pass, then the storage element is further programmed in the second pass so that the threshold voltage is increased to be within state B, as depicted by arrow 1710. The result of the second pass is to program the storage element into the state designated to store a logic “0” for the upper page without changing the data for the lower page. In both
In one embodiment, a system can be set up to perform full sequence writing if enough data is written to fill up an entire page. If not enough data is written for a full page, then the programming process can program the lower page programming with the data received. When subsequent data is received, the system will then program the upper page. In yet another embodiment, the system can start writing in the mode that programs the lower page and convert to full sequence programming mode if enough data is subsequently received to fill up an entire (or most of a) word line's storage elements. More details of such an embodiment are disclosed in U.S. Pat. No. 7,120,051, incorporated herein by reference in its entirety.
The programming process is a two-step process. In the first step, the lower page is programmed. If the lower page is to remain data 1, then the storage element state remains at state E. If the data is to be programmed to 0, then the threshold of voltage of the storage element is raised such that the storage element is programmed to state B′.
In one embodiment, after a storage element is programmed from state E to state B′, its neighbor storage element (WLn+1) in the NAND string will then be programmed with respect to its lower page. For example, looking back at
The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.
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